The Potential of Digested Sludge-Assimilating Microflora for Biogas Production from Food Processing Wastes

Round 1

Reviewer 1 Report

The article entitled “The potential of digested sludge microflora for biogas production from food processing wastes" presents the results of a study that aimed to assess the possibility of using biochar as an alternative to peat and as a carrier for beneficial microorganisms.

General note:

The subject of the study is interesting and topical, with scientific and practical importance.

The introduction is presented correctly, in accordance with the subject. Numerous scientific articles, in concordance with the topic of the study, were consulted.

The study methodology was modern and clearly presented, and appropriate to the proposed objectives.

The obtained results have been analyzed and interpreted correctly, in accordance with the current methodology.

The discussions are appropriate, in the context of the results, and were conducted compared to other studies in the field.

The scientific literature, to which the reporting was made, is recent and representative.

Below there are minor notes on the manuscript:

Line 26 perhaps keywords should be reconsidered so as not to repeat them in the title of the manuscript

Line 67-69 the underlining of the letters seems redundant

Were the results obtained statistically analyzed?  There is no information in the research methodology and no relevant references in the description of the research results. It is recommended that the authors provide this information to improve the study.

Author Response

Thank you very much for evaluating our manuscript. The changes in the revised manuscript are highlighted in yellow.

The article entitled “The potential of digested sludgeg microflora for biogas production from food processing wastes" presents the results of a study that aimed to assess the possibility of using biochar as an alternative to peat and as a carrier for beneficial microorganisms.

General note:

The subject of the study is interesting and topical, with scientific and practical importance. The introduction is presented correctly, in accordance with the subject. Numerous scientific articles, in concordance with the topic of the study, were consulted. The study methodology was modern and clearly presented, and appropriate to the proposed objectives. The obtained results have been analyzed and interpreted correctly, in accordance with the current methodology. The discussions are appropriate, in the context of the results, and were conducted compared to other studies in the field. The scientific literature, to which the reporting was made, is recent and representative.

Below there are minor notes on the manuscript:

Line 26 perhaps keywords should be reconsidered so as not to repeat them in the title of the manuscript

Keywords were reconsidered, according to your suggestion, in the revised manuscript (lines 26-27).

Line 67-69 the underlining of the letters seems redundant

The underlines were removed in the revised manuscript (lines 68-69).

Were the results obtained statistically analyzed?  There is no information in the research methodology and no relevant references in the description of the research results. It is recommended that the authors provide this information to improve the study.

Yes, the Student’s t-test was employed for biogas yield analysis (Fig. S1 which was newly made to show statistical results) and enzyme assay (Fig. 3). Information of statistical analysis was described in the revised manuscript (lines 195-197, 360-362 and legend for Tables S1-S5 and Fig. S1).

                                                                                   

Author Response File: Author Response.docx

Reviewer 2 Report

The present work presents a study on the biogas production capabilities of digested sludge-assimilating and biogas-yielding soil and enteric microflorae when used as seed cultures for biogas production from food processing wastes. The work is well presented, and the results are interesting, however, there are some aspects that should be addressed before it can be considered for publication. The most relevant are described as follows:

a) The motivation and scope of work is well established in the introduction. However, it is not possible to clearly identify the contribution, because there is no current discussion of the knowledge on the subject addressed. I suggest an extensive review of the subject.

b) In the methodology section the statistical methods used for the analysis of the experimental results used should be presented. What type is the experimental design used?

c) Although the study considers different types of food residues, they are very specific and do not cover the physicochemical characteristics of the wide variety of residues. How could the results found here be used to analyze/evaluate different types of waste? And what happens when we have mixtures of different types of waste?

d) The manuscript exhibits some grammatical errors and misspellings. It can certainly benefit from an extensive language revision.

No comments

Author Response

Thank you very much for evaluating our manuscript. The changes in the revised manuscript are highlighted in yellow.

The present work presents a study on the biogas production capabilities of digested sludge-assimilating and biogas-yielding soil and enteric microflorae when used as seed cultures for biogas production from food processing wastes. The work is well presented, and the results are interesting

however, there are some aspects that should be addressed before it can be considered for publication. The most relevant are described as follows:

a) The motivation and scope of work is well established in the introduction. However, it is not possible to clearly identify the contribution, because there is no current discussion of the knowledge on the subject addressed. I suggest an extensive review of the subject.

We compared biogas-yielding ability and hydrolase enzyme activity of our microflorae with published data for known anaerobic microflorae and discussed their usefulness as well as reconstructing sentences in the sections 3.1 and 3.4 in the revised manuscript (lines 217-245, 309-324)

b) In the methodology section the statistical methods used for the analysis of the experimental results used should be presented. What type is the experimental design used?

The Student’s t-test was employed for biogas yield analysis (Fig. S1 which was newly made to show statistical results) and enzyme assay (Fig. 3). Information of statistical analysis was described in the revised manuscript (lines 195-197, 360-362 and legend for Tables S1-S5 and Fig. S1).

c) Although the study considers different types of food residues, they are very specific and do not cover the physicochemical characteristics of the wide variety of residues. How could the results found here be used to analyze/evaluate different types of waste? And what happens when we have mixtures of different types of waste?

In the present study, our microflorae were found to produce biogas from various food residues whose pH and TOC/TN ratio range from 4.5 to 8.3 and 3.2 (nitrogen rich) to 170.8 (carbon rich), respectively. Therefore, we think that the microflorae appear to utilize a wide variety of other food residues as substrate for biogas production. Conversely, the microflorae could not assimilate orange peel, which contains limonene, an antibacterial terpene known to inhibit anaerobic digestion of citrus peel. Hence, some pretreatment (addition of biochar, for instance) would necessary to use orange peel for biogas production by removing limonene. As for successful anaerobic digestion of a garbage, a mixture of different types of food wastes, its TOC/TN ratio is basic and important parameter. Since it is reported that TOC/TN ratio of approximately from 10 to 30 is suitable for successful digestion a garbage, our microflorae might assimilate it and produce biogas. We added above discussion with quoting new references in the revised manuscript (lines 217-224, 226-230, 233-245).

 

d) The manuscript exhibits some grammatical errors and misspellings. It can certainly benefit from an extensive language revision.

Thank you for pointing out typos, omissions, and confusing sentences, which were corrected in the revised manuscript (Lines 2, 12, 18, 19-22, 55, 69, 75, 110, 115, 124, 127, 129, 167, 185-186, 187, 203, 204, 205, 206, 273-275, 278, 282-283, 294, 333, 337-338, 341, 346, 348, 355, 365, 366, 372, 374).

 

Author Response File: Author Response.docx

Reviewer 3 Report

This was a nice lab research dealing with a topic of current interest, i.e. the valorization of food processing wastes for the production of biogas. The following comments must be taken into consideration prior to the publication of the research.

-You must be specific about the origin of the FPWs in the materials and methods section. You must name each one of them in the text of this section (they appear in Table 1 but not in the text) and give their sources. As you have correctly stated in your article, "their composition varies across different manufacturers", so it is essential to provide the origin of the residues. Where they mixtures of different manufactures or did you get them from just one? You must mention the region of the industry or process plant from you've collected them. Also, the species from which they were obtained, if you know them. For example, do you know the fishes to which they belonged the fish bones? Do you know the varieties of grapes/oranges and the rest of the agro-foods?

-Page 3, Lines 127-132: Please mention which was the flow rate during the analysis of biogas. Also, mention how did you identify to which component corresponded to each chromatographic peak (CO2 etc). Did you use standards or mass spectrometer? In addition, how did you quantify them (creation of standard curves? percentage of the total?).

-Section 3.1: You must compare the biogas levels you found with more literature data. The text does not provide enough literature comparison to convince the reader about the potential of your processing. Add more literature data on the biogas levels reported for the same type of wastes using various microbial strains. Highlight for which wastes is your productivity antagonistic.

-Section 3.4: You must compare the enzyme activities obtained in your study to those reported in the literature. The referencing in this section is very poor. You must illustrate the potential of your findings as compared to literature data about the treatment of the same type of residues (e.g. orange peels, rice bran etc) using various microbial strains.

The text is readable and well-written at a scientific level. Minor English editing is suggested.

Author Response

Thank you very much for evaluating our manuscript. The changes in the revised manuscript are highlighted in yellow.

This was a nice lab research dealing with a topic of current interest, i.e. the valorization of food processing wastes for the production of biogas.

The following comments must be taken into consideration prior to the publication of the research.

-You must be specific about the origin of the FPWs in the materials and methods section. You must name each one of them in the text of this section (they appear in Table 1 but not in the text) and give their sources. As you have correctly stated in your article, "their composition varies across different manufacturers", so it is essential to provide the origin of the residues. Where they mixtures of different manufactures or did you get them from just one? You must mention the region of the industry or process plant from you've collected them. Also, the species from which they were obtained, if you know them. For example, do you know the fishes to which they belonged the fish bones? Do you know the varieties of grapes/oranges and the rest of the agro-foods?

Origin and providers of the FPWs and the species/variety name for fish, grape, and orange were described in the revised manuscript (lines 93-102), though their company name was not shown at their request.

-Page 3, Lines 127-132: Please mention which was the flow rate during the analysis of biogas. Also, mention how did you identify to which component corresponded to each chromatographic peak (CO2 etc). Did you use standards or mass spectrometer? In addition, how did you quantify them (creation of standard curves? percentage of the total?).

Methane, hydrogen, and carbon dioxide standard gases, were used for identification and quantification of the gas with creating their standard curves. Thermal conductivity detector was employed to detect peaks for methane, hydrogen, and carbon dioxide in biogas. The flow rate of carrier argon gas was 40 ml/min. Method for biogas analysis was replaced with more detailed description in the revised manuscript (lines 87-88, 133-142).

-Section 3.1: You must compare the biogas levels you found with more literature data. The text does not provide enough literature comparison to convince the reader about the potential of your processing. Add more literature data on the biogas levels reported for the same type of wastes using various microbial strains. Highlight for which wastes is your productivity antagonistic.

We replaced a discussion on biogas yield for same type of FPWs to more detailed one with adding several literatures (lines 217-245). In our study, orange peel was found recalcitrant for our microflorae. Published literatures suggest that limonene in the peel acts as antagonist for anaerobic digestion and biochar was one of useful absorbent to remove limonene and its addition stimulates digestion. We discussed this point with quoting the literatures (lines 226-230).

-Section 3.4: You must compare the enzyme activities obtained in your study to those reported in the literature. The referencing in this section is very poor. You must illustrate the potential of your findings as compared to literature data about the treatment of the same type of residues (e.g. orange peels, rice bran etc) using various microbial strains.

We compared enzyme activity of microflorae with data from published literatures, and found that our microflorae possess strong hydrolase activity in comparison to known anaerobic digester microflorae. We replaced a discussion on enzyme activity to more detailed one (lines 309-323).

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The authors have correctly addressed the observations, so in my opinion the manuscript can be accepted for publication.